A diesel engine which uses clean-burning DME (dimethyl ether), instead of light oil, as fuel is now attracting attention as means for reducing air pollution by diesel engines. DME fuel is a liquefied gas fuel unlike light oil as a conventional fuel. That is, DME fuel has a boiling point lower than that of light oil and vaporizes at room temperature, whereas light oil exists as a liquid at atmospheric pressure and temperature. Thus, in a diesel engine using DME fuel, there is a possibility that when DME fuel remaining in the injection system after stopping the engine leaks from a nozzle seat of a fuel injection nozzle into a cylinder of the engine and is evaporated therein and then the evaporated DME fuel fills the cylinder, abnormal combustion such as knocking occurs at the next start of the engine and the engine cannot be normally started, resulting in significant vibration and noise.
When the DME fuel remaining in the injection system of the DME fuel supply device after stopping the engine is retrieved into a tank by suction means such as an aspirator, it is possible to prevent abnormal combustion such as knocking at the next start of the engine caused by the DME fuel remaining in the injection system after stopping the engine. The aspirator does not suck the DME fuel using a suction force source such as a pump but creates a circulating flow of DME fuel using an injection pump for delivering DME fuel as a driving source and sucks the DME fuel by a suction force created by the flow of DME fuel.
However, it is difficult to suck all the DME fuel remaining in the injection system of the DME fuel supply device after stopping the engine into the tank with suction means such as an aspirator within a short period of time. This is because evaporated DME fuel cannot be sucked since the suction force of the aspirator is weak and since communication between the injection system and the fuel tank is shut off when the engine is stopped and the injection system is almost hermetically closed. That is, the DME fuel remaining in the injection system of the DME fuel supply device cannot be entirely retrieved before the DME fuel remaining in the injection system of the DME fuel supply device is entirely evaporated by residual heat of the engine or naturally.
Thus, it takes some time to retrieve the DME fuel remaining in the injection system of the DME fuel supply device entirely. It is, hence, impossible to retrieve the DME fuel remaining in the injection system of the DME fuel supply device entirely when the engine is stopped for a short period of time such as during idling stop at a signal crossing in an urban area, and abnormal combustion such as knocking may occur when the engine is restarted.
In a DME fuel supply device for a diesel engine using DME as fuel, DME fuel is supplied under pressure to a fuel gallery of the injection pump by a feed pump or the like to prevent it from evaporating, and highly-pressurized DME fuel is delivered under pressure to the fuel injection nozzle of the diesel engine through an injection pipe. Then, DME fuel overflowed from the fuel injection nozzle is delivered to a nozzle return pipe, and DME fuel overflowed from the fuel gallery is delivered to an overflow fuel pipe. The DME fuel delivered to the nozzle return pipe and the overflow fuel pipe is returned to the fuel tank through an overflow return pipe after having been cooled in a cooler or the like.
However, DME fuel is more likely to be affected by temperature by its nature than light oil fuel is, and the injection characteristics of DME fuel injected from the fuel injection nozzle are significantly changed by a small increase in temperature. Thus, when heat from the DME fuel supply device or the diesel engine is transmitted to the injection pipe and increases the temperature of the injection pipe, and the temperature of DME fuel delivered under pressure to the fuel injection nozzle is increased, the injection characteristics of DME fuel injected from the fuel injection nozzle may be unstable.
Also, immediately after the diesel engine has been stopped, the diesel engine is high in temperature and the injection pipe is also high in temperature. Thus, when DME fuel in a liquid state is filled into the injection pipe high in temperature from the fuel tank to restart the engine immediately, part of the DME fuel filled into the injection pipe may be evaporated by heat from the injection pipe and the evaporated DME fuel may prevent the DME fuel in a liquid state from filling the injection pipe.
It is necessary to correct the fuel injection amount with temperature using an electronic governor or the like to make the amount of DME fuel injected from the injection pump constant regardless of the changes in temperature of the DME fuel in the fuel gallery. Thus, in an injection pump provided with a mechanical governor, the amount of DME fuel injected from the injection pump cannot be constant regardless of the changes in temperature of the DME fuel in the fuel gallery and the injection characteristics of DME fuel cannot be stabilized.